Stamp the tag or short commit in a plain docker build (closes #211)

A plain `docker build .` stamped `dev`: `.dockerignore` left out `.git`
and the Dockerfile defaulted VERSION to `dev`. `.dockerignore` is now
the canonical one plus this repo's entries, sending `.git` without
`.git/config`. Given no build arguments, the builder stamps `git
describe --tags --always` and the commit and date from git, and fails
if `.git` is present but yields no version. The empty CHECK_EPOCH
refusal is gone so the plain build succeeds; the scripts still pass an
epoch. `script/version` now prints `git describe --tags --always
--dirty`, so make, the scripts and a plain build agree.

Model: opus-5-5

script/version

@@ -1,73 +1,28 @@
 #!/bin/sh
 # script/version: output the version string to bake into the binary.
-# Our own extension to scripts-to-rule-them-all, and the single source
-# of truth for the version: the Makefile's LDFLAGS call this rather
-# than carrying a hardcoded constant, which is what used to make every
-# local build claim to be 1.0.0-rc.1 regardless of git state.
+# Our own extension to scripts-to-rule-them-all. The Makefile's LDFLAGS
+# call this rather than carrying a hardcoded constant, which is what used
+# to make every local build claim to be 1.0.0-rc.1 regardless of git
+# state. script/docker and script/cibuild pass its output to the image
+# build, which otherwise runs the same `git describe` itself.
 #
-# The rules, in order:
+# The version is `git describe --tags --always --dirty`: the tag on a
+# tagged commit, tag-N-gHASH on a commit after one, the short commit
+# when no tag is reachable, each with a "-dirty" suffix when tracked
+# files have uncommitted changes. Untracked files are ignored: a stray
+# scratch file does not change what was compiled. Outside a git checkout
+# (release tarball, `go install`), or in one with no commits, it is
+# "dev".
 #
-#   HEAD is exactly on an annotated or lightweight tag
-#       -> that tag, with a leading "v" stripped
-#   anything else
-#       -> "dev-<12 chars of HEAD>"
-#   not a git checkout at all (release tarball, `go install`)
-#       -> "dev"
-#
-# Either of the first two gains a "-dirty" suffix when tracked files
-# have uncommitted changes, because a modified checkout of v1.0.0 is
-# not v1.0.0. Untracked files are ignored, matching `git describe
-# --dirty`: a stray scratch file does not change what was compiled.
-#
-# The "v" is stripped so that a `make` build and a goreleaser build of
-# the same tagged commit report the *same* string: goreleaser's
-# {{ .Version }} is the tag without the prefix, and the release archive
-# names are built from it. A tag named `v1.0.0` therefore produces
-# `vaultik 1.0.0`, matching `vaultik_1.0.0_linux_amd64.tar.gz`.
-#
-# Nothing here ever invents a version number. An untagged build says so
-# and names the commit it was built from; it does not round up to the
-# nearest plausible release.
+# Nothing here ever invents a version number.
 set -eu
 
 ROOT="$(cd "$(dirname "$0")/.." && pwd -P)"
 
-# Length of the commit prefix in a dev version. Matches
-# globals.ShortCommit, so `vaultik version` shows the same 12 chars in
-# its version line and its commit line.
-SHORT_LEN=12
-
 main() {
     cd "$ROOT"
-
-    if ! git rev-parse --git-dir >/dev/null 2>&1; then
-        echo "dev"
-        return 0
-    fi
-
-    dirty=""
-    if [ -n "$(git status --porcelain --untracked-files=no 2>/dev/null)" ]; then
-        dirty="-dirty"
-    fi
-
-    # --exact-match so a *descendant* of a tag is not reported as that
-    # tag. Plain `git describe --tags` would call a commit 40 patches
-    # past v1.0.0 "v1.0.0-40-gabc1234", and the leading token of that is
-    # a released version the build is not.
-    tag="$(git describe --tags --exact-match HEAD 2>/dev/null || true)"
-    if [ -n "$tag" ]; then
-        echo "${tag#v}${dirty}"
-        return 0
-    fi
-
-    sha="$(git rev-parse "--short=$SHORT_LEN" HEAD 2>/dev/null || true)"
-    if [ -z "$sha" ]; then
-        # A repo with no commits at all.
-        echo "dev"
-        return 0
-    fi
-
-    echo "dev-${sha}${dirty}"
+    version="$(git describe --tags --always --dirty 2>/dev/null || true)"
+    echo "${version:-dev}"
 }
 
 main "$@"